54 Q. What is mortal sin?
… It is our intention that makes the act we perform sinful or not. Let me explain. Suppose during Lent a person should mistake Friday for Thursday and should eat meat–that person would not commit a real sin, because it is not a sin to eat meat on an ordinary Thursday. He would commit what we call a material sin; that is, his action would be a sin if he really knew what he was doing. On the other hand, if the person, thinking it was Friday when it was really Thursday, ate meat, knowing it to be forbidden, that person would commit a mortal sin, because he intended to do so. Therefore, if what we do is not known to be a sin while we do it, it is no sin for us and cannot become a sin afterwards. But as soon as we know or learn that what we did was wrong, it would be a sin if we did the same thing again. In the same way, everything we do thinking it to be wrong or sinful is wrong and sinful for us, though it may not be wrong for those who know better. …
*56 Q. How many things are necessary to make a sin mortal?
A. To make a sin mortal three things are necessary: a grievous matter, sufficient reflection, and full consent of the will.
“Grievous matter.” To steal is a sin. Now, if you steal only a pin the act of stealing in that case could not be a mortal sin, because the “matter,” namely, the stealing of an ordinary pin, is not grievous. But
suppose it was a diamond pin of great value, then it would surely be “grievous matter.” “Sufficient reflection,” that is, you must know what you are doing at the time you do it. For example, suppose while you stole the diamond pin you thought you were stealing a pin with a small piece of glass, of little value, you would not have sufficient reflection and would not commit a mortal sin till you found out that what you had stolen was a valuable diamond; if you continued to keep it after learning your mistake, you would surely commit a mortal sin. “Full consent.” Suppose you were shooting at a target and accidentally killed a man: you would not have the sin of murder, because you did not will or wish to kill a man.
Therefore three things are necessary that your act may be a mortal sin:
(1) The act you do must be bad, and sufficiently important;
(2) You must reflect that you are doing it, and know that it is wrong;
(3) You must do it freely, deliberately, and willfully.